Millimeter Waves (mmWaves) are radio waves with wavelength in the range of 1 millimeter (mm)-10 mm, which corresponds to a radio frequency of 30 GigaHertz (GHz)-300 GHz. Per the definition by the International Telecommunications Union (ITU), these frequencies are also referred to as the Extremely High Frequency (EHF) band.
The mmWaves exhibit unique propagation characteristics. For example, compared with lower frequency radio waves, mmWaves suffer higher propagation loss, have a poorer ability to penetrate objects, such as buildings, walls, foliage, and are more susceptible to atmosphere absorption, deflection and diffraction due to particles (e.g., rain drops) in the air. On the other hand, due to the smaller wavelengths of the mmWaves, more antennas may be packed in a relatively small area, thereby allowing for the implementation of a high-gain antenna in small form factor.
The mmWaves have been less utilized than the lower frequency radio waves. A vast amount of spectrum is available in the mmWave band. For example, the frequencies around 60 GHz, which are typically referred to as the 60 GHz band, are available as unlicensed spectrum in most countries.
The technical field of indoor localization deals with developing systems and methods for localizing an object in an enclosed indoor area. The object can be a device that transmits and/or receives signals to/from some other device(s), or an entity without such a capability. The localizing refers to estimating the coordinates of an object in some pre-defined reference frame. Alternatively, localization can be framed as a proximity detection problem, where one aims to localize an object at a sub-area level, within a larger indoor area. A number of applications require precise indoor localization, such as locating people and resources in hospitals, warehouses, shopping malls, factories, to name a few. For example, a paradigm of technology-assisted living is built upon accurate localization.
A well-known solution for outdoor localization, known as Global Positioning System (GPS), is ineffective indoors because the electromagnetic waves transmitted from the satellites in the GPS constellation do not penetrate indoors.
A number of approaches in indoor localization require installing dedicated hardware in an indoor area. While this approach also has potential to yield accurate location estimates, it is undesirable because of the cost and the fact that a dedicated system is needed for localization task. An example of this approach is ultra-wide band (UWB) radio localization systems, commercially available, but relatively expensive and used only as a last resort. Other examples include systems based on lidar, radar or ultrasound, with usually high accuracy, but also high installation and maintenance cost. In the area of mmWave communication, the system described in CN102914762A discloses a mmWave-based indoor localization system. However, that system requires installation of a dedicated infrastructure operating at mmWave frequencies.
The infrastructure used for localization plays a major role in the selection of indoor localization method, along with the accuracy that can be achieved. For example, infrastructure-free indoor localization that does not require fingerprinting is a desirable approach from the cost and implementation perspectives. Such systems exploit an already existing infrastructure dedicated for some other tasks. A representative example is WiFi infrastructure, where the access points are dedicated for enabling wireless connectivity in a local area network. These methods usually rely on path loss modeling of propagation of the WiFi signals, see, e.g., U.S. Pat. No. 9,282,531. However, the principles of mmWave communications are very different from communication in the lower frequencies, and the existing methods suitable for WiFi signals are impractical for mmWave localization.